This invention relates to scanning radio receivers and, more particularly, to scanning radio receivers particularly useful on the frequencies assigned to the Public Safety Radio Services as well as other frequencies.
Scanning radio receivers are well known in the prior art and have found particular utility for the reception of radio signals on the frequencies assigned by the United States Federal Communications Commission to the Public Safety Radio Services. In the past, many such radio receivers used crystals as the tuning element to provide the necessary local oscillator signals and required the presence of one crystal for each frequency the receiver was capable of tuning. Examples of such receivers are those shown in U.S. Pat. Nos. 3,531,724 to G. H. Fathauer, 3,665,318 to S. J. Hoffman, et al., 3,714,585 to R. C. Koch, 3,725,788 to G. H. Fathauer, 3,794,925 to K. Imazeki, 3,801,914 to K. Imazeki, 3,821,651 to G. H. Fathauer, et al., 3,873,924 to G. H. Fathauer, 3,883,808 to J. E. Boone, 3,824,475 to P. W. Pflasterer, and 3,987,400 to G. H. Fathauer. Recently, scanning radio receivers using frequency synthesizing techniques have been provided which eliminated the need for a large number of crystals in radio receivers capable of being tuned to a large number of frequencies. Such receivers are shown in U.S. Pat. Nos. 3,937,972 to S. C. Snell, 3,961,261 to P. W. Pflasterer, 4,000,468 to J. R. Brown, et al., 4,027,251 to G. H. Fathauer, et al., 4,114,103 to P. W. Pflasterer, and 4,123,715 to G. H. Fathauer. There has also been provided by the prior art scanning radio receivers using frequency synthesizing techniques wherein the frequency synthesizing circuitry was controlled by the operation of a processing means such as a microprocessor. Exemplary radio receivers of this last mentioned type are disclosed in U.S. Pat. Nos. 3,962,644 and 4,092,594 both to W. Baker as well as the presently pending United States application Ser. Nos. 847,497 of G. H. Fathauer, et al., filed Nov. 1, 1979; 847,566 of G. H. Fathauer, filed Nov. 1, 1977 000,905 of W. L. Williamson, et al., filed Jan. 4, 1979; and 001,013 of A. Khan, et al., filed Jan. 4, 1979.
Scanning radio receivers making use of microprocessors to control the frequency synthesizing circuitry have many advantages over the scanning radio receivers known prior thereto. One of the foremost of these advantages is the very high degree of flexibility in control which may be included in such a radio. An example is the "search" mode of operation shown in the aforementioned U.S. Pat. Nos. 3,962,644 and 4,092,594 by which the radio may be successively and automatically tuned to adjacent ones of the Public Safety Radio Service frequencies until the receiver arrives at a frequency upon which a signal is received. Such operation was impractical with scanning radio receivers known prior thereto.
The search mode of operation described above while being a very advantageous feature has, however, certain disadvantages and problems associated therewith which have not been recognized and attacked. One particular disadvantage results from the fact that the frequencies which are assigned to the various ones of the public safety radio services, i.e., police, fire, telephone, utility, marine, etc., are in many instances intermixed between each other in what might appear to a user of the receiver as a rather arbitrary manner. Thus, if a user wants to search for and identify frequencies used by any particular service, such as police, he may either search the available frequencies or a portion thereof until he finds an active frequency and then by listening to the receiver determine whether the service of interest is using that frequency, or he may use a table showing him which frequencies are used by the service of interest and search only those frequencies. Both of these alternate methods are undesirably time consuming. In the first, all available frequencies must be searched and the listening operation performed, while in the second a separate search operation at each frequency assigned to the service of interest or covering each frequency band including frequencies assigned to the service of interest must be performed.
The Federal Communications Commission specifies that frequency modulation be used on all the frequencies assigned to the Public Safety Radio Services, but has specified that amplitude modulation be used on the frequencies assigned for other usages, particularly aircraft communication. Scanning radio receivers for use on the aircraft communication frequencies have previously been known, but those receivers were often capable of receiving only the aircraft communication frequencies and thus again only needed the facility for responding to signals of one modulation type, i.e., amplitude modulation. U.S. Pat. No. 3,725,788 to G. H. Fathauer discloses a radio receiver capable of scanning two Public Safety Radio Service frequencies and also of receiving conventional amplitude modulated broadcast signals, but needing no circuitry to provide a scanning operation in connection with amplitude modulated signals.
The problems involved in producing a single scanning receiver capable of receiving both the public safety and aircraft communication frequencies have not been clearly recognized and solutions have not been evident. A receiver including circuitry for demodulating both amplitude modulated and frequency modulated signals must perform under the operating conditions imposed by the environment of a scanning radio receiver.
To elaborate somewhat, in its scanning mode of operation a scanning radio receiver is successively tuned to different frequencies previously selected by the user until it arrives at a frequency at which a signal above a minimum signal strength is received. During the time it is tuned to any individual frequency, it must ascertain whether a signal of sufficient strength is present at that frequency, and this is usually done by evaluating some characteristic (such as high frequency noise content) of the demodulated signal. The scanning process is typically performed at the rate of as many as ten or more frequencies per second. Thus, if aircraft communication and public safety frequencies are to be scanned successively and intermixed in the order of scanning, it is necessary that some mechanism be provided for altering the demodulation operation of the receiver between frequency and amplitude demodulation depending on the form of modulation used at the frequency to which the receiver is tuned at any instant.